The present invention relates to a sensor for determining the UV fraction of a radiation which strikes the sensor. In a further aspect, the invention relates to a sensor for determining the UV irradiance.
The invention also relates to a measuring device comprising an optical fiber sensor for determining the UV fraction and if necessary the UV irradiance, together with a corresponding method.
All materials of organic origin are at risk if exposed to incorrect illumination, owing to the chemical reactions initiated by exposure to light. A typical reaction is photo-oxidation, in other words the formation of free radicals following the absorption of a photon by an organic molecule and the subsequent binding of the radical with an oxygen molecule. The peroxide radical formed in this way gives rise to a series of chemical reactions which lead to the progressive deterioration of the organic material. The damage caused to the organic material is cumulative and irreversible.
This problem is particularly serious in the field of the fine arts, since excessive illumination of the works displayed in museums, art galleries and other exhibition sites causes irreversible damage.
It has therefore been necessary to set limits of illumination in the museum environment, specified in precise standards. By way of example, the limits stipulated in the Italian UNI standards (CTI E02.01.304.0) are shown in the following table.
The term illuminance (or illumination) denotes the luminous intensity of the light incident on a unit of surface area. This value is measured in lux (lx), and is equal to the luminous flux per unit of surface area: lx=lm/m2, where lm (lumen) is the unit of measurement of the luminous flux. The intensity of the UV radiation incident on the unit of irradiated surface area (measured in W/m2) is called the UV irradiance. The UV fraction is defined as the ratio between the UV irradiance and the illuminance and is measured in W/lm. This provides an indication of the quantity of UV radiation present in the total radiation incident on the unit of surface area.
The object of the present invention is to provide a sensor, a measuring device and a method of measurement for the determination of the UV fraction.
The object of an improved embodiment of the present invention is to provide a sensor which can also be used to measure the UV irradiance.
Essentially, the sensor according to the invention comprises a photochromic transducer consisting of a material with characteristics of reversible photochromism, for example a fulgide, whose absorbance varies as a function of the UV fraction of the radiation incident on the photochromic material, and at least a first optical path which guides a light signal through said photochromic material and collects a return light signal whose value is proportional to the level of absorbance of the photochromic material. The optical path preferably consists of an optical fiber or a bundle of optical fibers. The measurement may be made by reflection or by transparency. In the first case, the optical fibers for transmitting the radiation and for capturing the return signal are located on the same side of the photochromic material and associated with a diffusing or reflecting surface located on the opposite side. The possibility of positioning the outgoing and return fibers facing each other, with the photochromic material placed between them, is not excluded.
As will be explained below, the transducer assumes a coloration and consequently a level of absorbance which varies with the UV fraction of the radiation incident on it. It is therefore possible to interrogate the sensor by sending a light signal which passes through the fulgide and by capturing the attenuated signal, whose degree of attenuation provides a measurement of the UV fraction.
In an improved embodiment of the invention, the sensor may also comprise a further optical path for guiding toward the fulgide transducer a light radiation having characteristics such that a decoloration of the photochromic material is caused. The behavior of the fulgide material in the time interval following complete decoloration, under the effect of the external radiation incident on it, depends on the UV irradiance, in other words on the intensity of the ultraviolet radiation incident on it. It is therefore possible, by means of the same light signal used for the determination of the UV fraction, to determine the shape of the curve of attenuation with time and to derive the UV irradiance from this by the method described below.
With a sensor of this type it is therefore possible to implement a measurement method comprising the steps of: a) making the radiation to be measured strike a photochromic material, in other words a fulgide; b) making a light signal pass through the fulgide; c) determining the UV fraction of the radiation as a function of the attenuation of the light signal as it passes through the fulgide.
In its improved embodiment, the sensor according to the invention can be used to implement a method of measuring the UV irradiance, comprising the following steps: a) striking the fulgide transducer with a light beam having a wavelength such that it causes a decoloration of the fulgide, until its decoloraton is achieved; b) making a light signal pass through the fulgide while the latter is being struck by the radiation to be measured; c) detecting the variation with time of the attenuation of the light signal passing through the fulgide transducer, thus determining an attenuation curve; d) determining the UV irradiance of said radiation from the slope of the attenuation curve.
The various steps described above can be repeated at more or less regular intervals, to obtain a plurality of measurements over time and consequently the variation with time of the UV irradiance. When the fulgide transducer has reached the stable state, the UV fraction is determined by means of said transducer.
Further advantageous characteristics of the sensor and method according to the invention, and of the device which makes use of these, are indicated in the attached claims.